A major problem in the furniture industry, both in terms of raw material usage and environmental effects, concerns the solvent content of nitro cellulose lacquer-based liquid coating compositions which are used as a protective coating for wood pieces of furniture. The resinous material contained in such nitrocellulose lacquer compositions is typically dissolved in an organic solvent provided with a viscosity suitable for spraying. This is required because it has been found that at each stage of the process for atomizing and conveying a resinous material in liquid form to a substrate, the liquid resists high speed deformation. Organic solvents are added to the resinous liquid because they have the effect of separating the molecules of resinous material and facilitating their relative movement making the solution more deformable at high speeds and therefore more susceptible to atomization. Substantial effort has been expended to reduce the volume of liquid solvent components in preparing high solid coating compositions, such as nitrocellulose lacquer-based compositions, but such compositions or formulations still typically contain as much as 60% or more by volume of liquid solvent components.
The problem with such a high volume content of liquid solvents in nitrocellulose coating compositions is that during handling, atomization or deposition of the coating composition, the solvents escape and can become air contaminants if not properly trapped. Once the coating composition is applied to a substrate, its solvents escape from the film by evaporation and such evaporated solvents can also contaminate the surrounding atmosphere. Additionally, since most solvents react with oxidants, pollution problems of toxicity, odor and smog may be created. Attempts at overcoming such environmental problems are proven to be costly and relatively inefficient.
The problem of high volume content of liquid solvents in coating compositions has been addressed, for example, in U.S. Pat. No. 4,923,720 to Lee et al. This patent discloses a method and apparatus for the production of a coating formulation in which a substantial amount of the liquid solvent component is removed and replaced with a supercritical fluid such as supercritical carbon dioxide which functions as a diluent to enhance the application properties of the coating formulation. The supercritical carbon dioxide and some liquid solvent material, e.g., about two thirds less than is required in other coating compositions, are intermixed with polymeric solids to form a coating material solution or formulation having a viscosity which facilitates atomization through an airless-type coating dispenser. As the coating material formulation is discharged from the dispensing devices toward a substrate, the supercritical carbon dioxide "flashes off" or vaporizes to assist in atomization of the high solids coating composition and to reduce drying time of the composition on the substrate. Such coating material formulation has the advantage of substantially reducing the adverse environmental effects caused by coating compositions having a high solvent content.
It has been observed that in the particular application of applying nitrocellulose lacquer-based liquid coating compositions onto wood pieces of furniture, problems have arisen as the result of the formation of a solid precipitate within the coating material formulation which plugs or blocks the dispensing system. As disclosed in the Lee et al U.S. Pat. No. 4,923,720, the supercritical carbon dioxide and liquid coating composition are supplied separately and intermixed within a "loop" or flow path which transmits the resulting coating material formulation to dispensers for discharge onto a substrate. The solid precipitate formed in the course of combining the supercritical carbon dioxide with the liquid coating composition to produce the nitrocellulose lacquer-based formulation clogs or blocks the valve or injector device which introduces the supercritical carbon dioxide into the loop. As a result, an insufficient volume of supercritical carbon dioxide is discharged into the loop and the resulting coating material formulation has an improper ratio of supercritical carbon dioxide to liquid coating composition. If sprayed onto a substrate with such improper ratio, the coating material formulation might not properly atomize and/or produce an unacceptable coating on the substrate.
The problem of forming a solid precipitate in the course of applying nitrocellulose lacquer-based formulations which include a supercritical fluid diluent has been addressed in U.S. Pat. No. 5,105,843 to Condron et al. This patent recognizes that the blockage or clogging problem occurs at the point of injection of the supercritical fluid into the system, and proposes the use of an isocentric, low turbulence injector to prevent the dissolved solids within the liquid coating composition from coming out of solution. This injector comprises a first tube carrying supercritical fluid which is concentrically disposed within a larger diameter second tube carrying the liquid coating composition. The liquid coating composition is directed in a laminar flow through the second tube, and the supercritical fluid is then introduced as a "core" fluid, i.e., at the center of the flow of liquid coating composition surrounding the first tube, so that the liquid coating composition and supercritical fluid are intermixed with a minimum of turbulence. This system therefore requires a particular construction of an injector for both the liquid coating composition and supercritical fluid, and the flow rates at which each material are combined with one another must be carefully controlled. In the event these flow rates are increased, which could be required if a relatively large number of coating dispensers must be supplied with the coating material formulation, it is believed that problems of turbulent flow may arise causing the formation of the undesired, solid precipitate.